Poweramp valves, not really matched.....

[AdamB]

I bought some JJ 6L6's off Hotrox, which i've always pretty good. Anyway, i ordered a matched pair, and when i put them in my Egnater to bias them, i checked the first one, set it at 35mv, checked the other, and it was only 30mv.

Now 5mv's apart is quite a bit for a 'matched' pair, right? I thought 3mv's or less was desirable.

Will they work out of phase and give me a weird sound? Or just not a good as sound as it could be?

Thanks

[Twinfan]

I reckon that's close enough for a matched pair??

[AdamB]

Ah, ok, i was doing a bit of reading up, and saw 3mv's or less. Think i'm just being a bit paranoid really

[gwEm]

mate! thats not at all bad you know!! i've seen far far worse 'matched pairs'

[AdamB]

Cool, cool, i'm glad to hear it :D

So i won't get any weird crossover sounds etc

[gwEm]

So i won't get any weird crossover sounds etc

no :)

[AdamB]

Thanks man

[hamfist]

Good matching is usually referred to as within 5%. This is the figure that Mike at www.Kcanostubes.com uses. Watford valves claim that their equipment can match within 1% but, cunningly, do not specifically claim that matched sets from them will fall within this.
 thetubestore also match within 5% but, interestingly, also write :-

"We match tubes to be within 5% of measured plate current (that is, the tubes won't differ from each other by more than 5%, or 1 milliamp in 20). Despite what many people may say, this is about as accurate as power tubes can be matched. Tubes will always test differently from day to day because of fluctuations in the power supply and "drift" in the tube over time as it ages. When you consider that most electrical components in a modern amplifier are built with a tolerance of 10%, this is more than accurate enough. By contrast, common practice in the tube industry for the last fifty years has been to match power tubes to within 25%."

[jpfamps]

I've installed valves matched by most of the popular vendors, and would have to say the matching a generally OK although I have had some sets that are way out.

Two problems when matching valves:

1) The valves are usually matched at a lower plate voltage than is used in guitar amps. 400 V for large bottle valves if you are lucky, but often 250 V.

2) The current draw tends to drift somewhat during the first few hours of life, thus to accurately match valves you should allow then to "burn in" for several hours, which of course adds cost.

The accuracy of the matching will also depend on the number of valves you have. If you order 100 new valves you will be able to match up several pair and perhaps few quads within a 1 ma. If you only have a few NOS valves then you might be lucky to get them within 5 mA.

The best way to match valves is in the equipment you are using them in, which is impractical for most end users.

Regardless the small difference you are seeing is fine.

[HTH AMPS]

I was about to wade-in with my size-10 doc martins, but JPF said exactly what I'd planned to say.

I've asked whether 'matched' sets are burned in for 24 hours before and was given evasive answers, so I took it as a 'no'.  They play a numbers game - most of the time the valves won't drift too much during burn in, so it's easier to quickly match them up and deal with the odd set that drift miles appart when it occurs.

There are guys in the US like Lord Valve and KCA who burn in valves and match them up in guitar amps so you know you're getting a matched set that will be spot-on in an actual amp.  However, you've got the cost of importing them to the UK and if you have problems it's a major pita.

The sets I've got from Hotrox have always been all over the map (upto 10mA apart) and Watford 'used' to be really good with 1mA matching, but not anymore.  Come to think of it, I've never had a closely matched set of valves for ages now.

I buy on ebay these days, the guys selling on there are as good as anywhere (imo).

The fortunate thing with guitar amps is that some mismatch isn't a bad thing.  The major problem is that the more unmatched the valves become, the more hum you get.

[AdamB]

Thanks everyone